The milestone doesn't mean 1 billion people are visiting the site every day — yet. But it does highlight Facebook's constantly impressive growth and is another reminder the site isn't showing any signs of slowing down.

Last quarter, Facebook reported 968 million daily active users in June alone, up 17 percent year over year. That's also not counting users of Facebook-owned services like Instagram and WhatsApp.

Facebook's made a concerted effort to keep its user base strong by breaking into emerging Internet markets early. That principle is what's driving Zuckerberg's Internet.org project, which offers a slimmed-down version of the Internet to developing countries.

And the push seems to be working out well. As Quartz reported in February, Facebook users in some countries had no idea they were connected to the broader Internet. For them, there was only Facebook.